1. Field of the Invention
This invention relates generally to a device and method that measures the biomechanical properties, the stress-strain relationship of the cornea together with the physical properties of a mammal's anterior chamber of the eye in vivo. The present innovation also applies to methods to monitor the intraocular pressure of a mammal eye.
2. Prior Arts
Cornea contributes more than 60% of refractive power of the vision system thus most of the refractive surgeries for vision corrections were performed on the cornea. Current diagnostic instruments used in the eye clinics are able to measure the radius of curvature, the astigmatism and the thickness of cornea before, during and after the refractive surgeries. With all these measurement and the wave front technology, ultimate refractive surgical outcome was guaranteed. However the promises to enhance the predictability of surgical outcomes were not met, debilitating visual complaints are reported. Several studies indicate that bio-mechanical properties, the stiffness or the elasticity of the cornea plays a very important role of the stability of the cornea after refractive surgery.
The possibility of ectasia, the bulging of the cornea, after the ablation type of laser surgeries, for example LASIK, also demonstrates the unsatisfactory outcome of laser refractive surgeries due to the lack of biomechanical properties of corneas. Although the incident rate of ectasia appears low now, LASIK has only been used in clinical practice in recent years and the long-term results are unknown. Recent studies showed that the corneal thickness alone can not explain the ectasia incidents and bio-mechanical properties of the cornea are being suggested. Thus screening of the cornea for its biomechanical defects together with the cornea thickness radius of curvature and other physical parameters before LASIK type procedures is becoming more urgent.
Direct measurement of in vivo bio-mechanical properties in the human eye has been rare until recently. Grabner used video topographic method to measure the cornea distortion induced by an indenter which was powered by a micro-precision motor. This method requires contact of the patient's cornea with a pointed indenter and is not practical in the clinical setting. This device is not able to provide other physical parameters of the cornea. Luce measured the hysteresis of the cornea rebound due to the impact of an expanded air pulse to index the cornea biomechanical properties. Because it is based on the reflection of limited points of light, the method suffers from the same reliability problem when the surface of the cornea is irregular as the non-contact tonometer suffers. Furthermore, Luce's inventions, including U.S. Pat. No. 6,875,175, can not provide the physical parameters of the cornea, for example: radius of curvature and the thickness of the cornea, etc.
Another important clinical issue related to the lack of in vivo data of the cornea biomechanical properties has been demonstrated in the clinical monitoring of the intraocular pressure, an important indicator to control the progress of glaucoma. Current intraocular pressure measurement, no matter contact or non-contact tonometries, requires the measurement via the cornea. Both methods are based on the flattening of the cornea by external applanation forces and interpretation of the rebound force of cornea.
Maklakov was the first one who developed the impression tonometer in 1885. In this device, a plunger of a known weight is allowed to rest against the eye with the patient in a supine position. The area of contact is determined with a dye, and the weight divided by the area of contact gives the pressure.
The modern tonometer was invented in the 1950's by Goldmann (U.S. Pat. No. 3,070,997). In this device, a small biprism is pressed against the cornea. The cornea is prepared by applying a topical anesthetic and a dye which is illuminated by a slit lamp. The image of the glowing tear film around the edge of the prism is split by the biprism such that when just enough pressure is applied to applanate the cornea to the diameter of the prism, the half-images of the glowing ring become perfectly aligned. Great skill is required in order to obtain accurate measurements and the disturbances of the intraocular pulse must be averaged out visually. In general, tonometry measurements vary about 10% from examiner to examiner.
Grolman disclosed a non-contact tonometer, U.S. Pat. No. 3,585,849, which operates by impacting an air pulse at the cornea. The time for the cornea to return to the predetermined shape due to the impacting of the air pulse was measured via the optical reflection of a point light source upon the cornea and correlated to the intraocular pressure.
This non-contact device has several disadvantages: the reading is inaccurate on irregular cornea surfaces due to its single point probing system; it overestimates the tension with increased tear film and ocular misalignment and the reading is variable due to the ocular pulses in the eye. The device is of little clinical value above 30 mmHg due to lack of accuracy. In spite of the difficult clinical technique, possible damage to the cornea, chances of communicating disease and questionable accuracy, the contact tonometer, particularly the Goldmann tonometer, is the still gold standard of the measurement because of the problems with non-contact tonometers.
From the very beginning, both intraocular pressure measurements are known to vary as a function of corneal thickness, radius of curvature and rigidity and all of which may vary from patient to patient. Recently, more quantitative information of this interference is emerging clinically and theoretically.
The Ocular Hypertension Treatment Study (OHTS) reported the significant effect of Central Cornea Thickness (CCT) on intraocular measurement, CCT measurement was performed and used to correct the subsequent measurement by Goldmann applantation tonometer or non-contact tonometer with an empirical correlation formula. Grolman, U.S. Pat. No. 5,474,066, and Hyman, U.S. Pat. No. 6,113,542, proposed including the cornea thickness in the non-contact or contact tonometer measurement. In practice, variations were already reported using this approach, with non-contact tonometer measurement garnering the most variation.
This kind of variation was due to the fact that the thickness of the cornea alone does not account for the total resistance and effect of the cornea and thus the inaccuracy of the intraocular pressure.
These sources of inaccuracy of the tonometer measurement and the variability of corneas are accentuated in the more than 1 million patients per year who undergo corneal refractive surgery.
Recently, Orssengo and Roberts quantified the correlation between the cornea thickness, the curvature, the elasticity of the cornea and the intraocular pressure readings. Their finding together with some simulation calculation reemphasizes that the cornea factor needs to be isolated from any measuring method of intraocular pressure.
Thus, to improve the refractive surgeries and the intraocular pressure monitoring, there is a great national and international need to develop a comprehensive device to measure the biomechanical properties of the cornea.